i know there are a ton of posts on misfires but I thought I would start my own thread for my specific case. Ok so here is what I have done already. New maf new o2's new plugs wires cap and rotor new pcm new valve cover gaskets. I have always had the cylinder 4 misfire and had a bunch of codes with it before. I have since fixed all the other codes. I have a random coolant leak that I cannot see anywhere and a leak down test proves the the system is in fact leaking but still no drips on the ground or anywhere on the motor. It's also running a bit lean 8.9% bank 1 2.7% bank 2. It was 25% on both banks and I fixed the major leaks where it was the valve covers and the top of the throttle body. I suspect my lower intake is leaking the coolant and making the motor run lean. Also I checked the spider injector assembly and it is in good condition. I was wondering if anyone could point me in the right direction and if you think the coolant leaking through the intake (if that is the case) could be causing this misfire
99 Yukon cylinder 4 missfire
- Thread starter Cronkite
- Start date
Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.
east302
Full Access Member
Leaking intake manifold gaskets could cause your misfire issue. How does the oil look?
Are you seeing any leaks between the block and lower intake during a pressure test? To check for a lower intake leak, I've read where you remove the PCV and crankcase breather tube, plug the holes in the valve cover and check for vacuum through the oil dipstick when the engine is running. If you have vacuum, the gaskets are suspect.
Sent from my iPhone using Tapatalk
Are you seeing any leaks between the block and lower intake during a pressure test? To check for a lower intake leak, I've read where you remove the PCV and crankcase breather tube, plug the holes in the valve cover and check for vacuum through the oil dipstick when the engine is running. If you have vacuum, the gaskets are suspect.
Sent from my iPhone using Tapatalk
I will take a look at that and see what I come up with. I have not seen anything leaking on the lower intake plenum. But it's really difficult to see in there with everything in the way. And the oil is good. And there isn't any oil in the coolant either
Ok so I started tearing my motor apart after running the compressions test and finding cylinder 4 was low. Found that a big coolant leak in all 4 corners of the lower intake was present and also found a ton of sludge everywhere. I just bought the truck last year so of course I don't know how well it was maintained but I have a good idea of how well they didn't take care of it lol. Here are some before and after pics. Turns out that with a plastic bristle brush and some sea foam and 2 hours of scrubbing it came right off lol....... I will continue to post the progress photos as I go. Hopefully this is going to fix my issue. The heads are at a local machine shop getting cleaned and inspected. Here's for crossing fingers and hoping nothing is seriously wrong with them
RicanGM
Full Access Member
Looking good
bigfootchiro
Full Access Member
If only I had the time to do all of that! Would love to take mine apart and clean it. Let us know how it runs after you're all done! Looks great
The Yukon is basically finished with the major repairs. On top of everything I have mentioned I also replace both front wheel hubs and axles and calipers. Plus all the u joints trans mount, fixed a leak in my t case. And restored the headlights. I still need to swap the pitman arm and tie rods inner and outer. But yes the truck is running better than ever. I just need to run it to my friends shop to check and adjust cam retard
It runs great! Better than ever. Funny thing when you buy something that doesn't quite run right and drive it like that for awhile thinking you are just using it for the mean time, then decide to fix it and now that it's running the way it is supposed to be it feels weird like something is wrong
